Heat the oven to Gas 4, 180°C, 350°F.
Cook the cauliflower florets in boiling water for 5 minutes, remove and drain, leave in the colander to continue to drain.
Melt the butter in a small saucepan, add flour and mix to a smooth paste. Remove from the heat, add a small amount of milk and mix until smooth, continue until all the milk is incorporated. Add seasoning, return to the heat, stirring constantly, and bring to a gentle simmer for 1 minute.
Add cheese, reserving some, and mix. Pour a little of the sauce into an oven-proof dish, add cauliflower and spoon over the remaining sauce. Sprinkle with the reserved cheese and place in the oven for 10 minutes until bubbling and golden.